1. Field of the Invention
This invention relates to cellulose fiber reinforced cement composite materials using loaded cellulose fibers, including formulations, methods of manufacture and final products with improved material properties relating to the same.
2. Description of the Related Art
Ordinary Portland cement is the basis for many products used in building and construction, primarily concrete and steel reinforced concrete. Cement has the enormous advantage that it is a hydraulically settable binder, and after setting it is little affected by water, compared to gypsum, wood, wood particle boards, fiberboard, and other common materials used in building products. This is not to say that water has no effect on cement. Some dissolution of chemical components does occur when cement is saturated with fresh water, and these can be transported and re-deposited in different places if the cement is once again dried.
Asbestos Fiber Cement Technology
About 120 years ago, Ludwig Hatschek made the first asbestos reinforced cement products, using a paper-making sieve cylinder machine on which a very dilute slurry of asbestos fibers (up to about 10% by weight of solids) and ordinary Portland cement (about 90% or more) was dewatered, in films of about 0.3 mm, which were then wound up to a desired thickness (typically 6 mm) on a roll, and the resultant cylindrical sheet was cut and flattened to form a flat laminated sheet, which was cut into rectangular pieces of the desired size. These products were then air-cured in the normal cement curing method for about 28 days. The original use was as an artificial roofing slate.
For over 100 years, this form of fiber cement found extensive use for roofing products, pipe products, and walling products, both external siding (planks and panels), and wet-area lining boards. Asbestos cement was also used in many applications requiring high fire resistance due to the great thermal stability of asbestos. The great advantage of all these products was that they were relative lightweight and that water affected them relatively little, since the high-density asbestos/cement composite is of low porosity and permeability. The disadvantage of these products was that the high-density matrix did not allow nailing, and methods of fixing involved pre-drilled holes.
Although the original Hatschek process (a modified sieve cylinder paper making machine) dominated the bulk of asbestos cement products made, other processes were also used to make specialty products, such as thick sheets (say greater than about 10 mm which required about 30 films). These used the same mixture of asbestos fibers and cement as with the Hatschek process, although sometimes some process aid additives are used for other processes. For example, fiber cement composites have been made by extrusion, injection molding, and filter press or flow-on machines.
Two developments occurred around the middle of the last century that had high significance to modem replacements of asbestos based cement composites. The first was that some manufacturers realized that the curing cycle could be considerably reduced, and cost could be lowered, by autoclaving the products. This allowed the replacement of much of the cement with fine ground silica, which reacted at autoclave temperatures with the excess lime in the cement to produce calcium silica hydrates similar to the normal cement matrix. Since silica, even when ground, is much cheaper than cement, and since the autoclave curing time is much less than the air cured curing time, this became a common, but by no means universal manufacturing method. A typical formulation would be about 5-10% asbestos fibers, about 30-50% cement, and about 40-60% silica.
The second development was to replace some of the asbestos reinforcing fibers with cellulose fibers from wood. This was not widely adopted except for siding products and wet-area lining sheets. The great advantage of this development was that cellulose fibers are hollow and soft, and the resultant products could be nailed rather than by fixing through pre-drilled holes. The siding and lining products are used on vertical walls, which is a far less demanding environment than roofing. However, cellulose reinforced cement products are more susceptible to water induced changes, compared to asbestos cement composite materials. A typical formulation would be about 3-4% cellulose, about 4-6% asbestos, and either about 90% cement for air cured products, or about 30-50% cement and about 40-60% silica for autoclaved products.
Asbestos fibers had several advantages. The sieve cylinder machines require fibers that form a network to catch the solid cement (or silica) particles, which are much too small to catch on the sieve itself. Asbestos, although it is an inorganic fiber, can be xe2x80x9crefinedxe2x80x9d into many small tendrils running off a main fiber. Asbestos fibers are strong, stiff, and bond very strongly with the cement matrix. They are stable at high temperatures. They are stable against alkali attack under autoclave conditions. Hence, asbestos reinforced fiber cement products are themselves strong, stiff (also brittle), and could be used in many hostile environments, except highly acidic environments where the cement itself is rapidly attacked chemically. The wet/dry cycling that asbestos roofing products were subjected to, often caused a few problems, primarily efflorescence, caused by the dissolution of chemicals inside the products when wet, followed by the deposition of these chemicals on the surfaces of the products when dried. Efflorescence caused aesthetic degradation of roofing products in particular, and many attempts were made to reduce it. Because the matrix of asbestos reinforced roofing products was generally very dense (specific gravity about 1.7), the total amount of water entering the product even when saturated was relatively low, and the products generally had reasonable freeze thaw resistance. If the density was lowered, the products became more workable (for example they could be nailed) but the rate of saturation and the total water absorption increased and the freeze thaw performance decreased.
Alternative Fiber Cement Technologies
In the early 1980""s, the health hazards associated with mining, or being exposed to and inhaling, asbestos fibers started to become a major health concern. Manufacturers of asbestos cement products in the USA, some of Western Europe, and Australia/New Zealand in particular, sought to find a substitute for asbestos fibers for the reinforcement of building and construction products, made on their installed manufacturing base, primarily Hatschek machines. Over a period of twenty years, two viable alternative technologies have emerged, although neither of these has been successful in the full range of asbestos applications.
In Western Europe, the most successful replacement for asbestos has been a combination of PVA fibers (about 2%) and cellulose fibers (about 5%) with primarily cement (about 80%), sometimes with inert fillers such as silica or limestone (about 10-30%). This product is air-cured, since PVA fibers are, in general, not autoclave stable. It is generally made on a Hatschek machine, followed by a pressing step using a hydraulic press. This compresses the cellulose fibers, and reduces the porosity of the matrix. Since PVA fibers can""t be refined while cellulose can be, in this Western European technology the cellulose fiber functions as a process aid to form the network on the sieve that catches the solid particles in the dewatering step. This product is used primarily for roofing (slates and corrugates). It is usually (but not always) covered with thick organic coatings. The great disadvantage of these products is a very large increase in material and manufacturing process costs. While cellulose is currently a little more expensive than asbestos fibers at $500 a ton, PVA is about $4000 a ton. Thick organic coatings are also expensive, and hydraulic presses are a high cost manufacture step.
In Australia/New Zealand and the USA, the most successful replacement for asbestos has been unbleached cellulose fibers, with about 35% cement, and about 55% fine ground silica, such as described in Australian Pat. No. 515151 and U.S. Pat. No. 6,030,447, the entirety of which is hereby incorporated by reference. This product is autoclave cured, as cellulose is fairly stable in autoclaving. It is generally made on a Hatschek machine, and it is not usually pressed. The products are generally for siding (panels and planks), and vertical or horizontal tile backer wet area linings, and as eaves and soffits in-fill panels. The great advantage of these products is that they are very workable, even compared to the asbestos based products, and they are low cost.
However, cellulose fiber cement materials can have performance drawbacks such as lower resistance to water induced damages, higher water permeability, and higher water migration ability (also known as wicking) compared to asbestos cement composite materials. These drawbacks are largely due to the presence of water conducting channels and voids in the cellulose fiber lumens and cell walls. The pore spaces in the cellulose fibers can become filled with water when the material is submerged or exposed to rain/condensation for an extended period of time. The porosity of cellulose fibers facilitates water transportation throughout the composite materials and can affect the long-term durability and performance of the material in certain environments. As such, conventional cellulose fibers can cause the material to have a higher saturated mass, poor wet to dry dimensional stability, lower saturated strength, and decreased resistance to water damage.
The high water permeability of the cellulose reinforced cement materials also results in potentially far greater transport of soluble chemicals within the product. The soluble chemicals can then re-deposit on drying, either externally, causing efflorescence, or internally, in capillary pores of the matrix or fiber. Because the materials are easier to saturate with water, the products also are far more susceptible to freeze/thaw damage. However, for vertical products, or eaves and soffit linings, and for internal linings, none of these water-induced disadvantages are very relevant.
To summarize, the replacement of asbestos in Europe has been largely by air cured fiber cement products, using PVA fibers, and pressed after forming in the green state. The primary problem with this technology is increased material and manufacturing cost. The replacement of asbestos in USA and Australia/New Zealand has been largely by autoclaved fiber cement products, using cellulose fibers, and formed with lower density without pressing. The primary problem with this technology is increased rate, and quantity of water absorption into the product when wet.
Several prior art references disclose the use of fibrous materials in cement products, as well as various processes for treating the fibrous materials. However, most of these references are directed to increasing the bond strength of the fibrous material to the cement, rather than addressing the water and moisture related issues of cellulose and/or other fibers. Many of these references disclose methods for treating the fibrous material by mineralization, thereby forming precipitates on the surface of the fibrous material. For example, U.S. Pat. No. 5,795,515 describes an air-cured product including a high percentage of cement (e.g., 70-80%) and cellulose fibers which have been mineralized by pretreating the fibrous material with aluminum sulfate, and subsequently adding amorphous silica to the fibers. Similarly, U.S. Pat. No. 2,377,484 discloses woody and vegetable fibers, such as excelsior, which are treated with sodium silicate and calcium chloride to precipitate calcium chloride on the fibers.
The purpose of mineralizing the fibers in these and other references is to provide a coating which serves to bond the fibers with the cement. Other references also relate to increasing the bond strength between the fibrous materials and the cement. For example, U.S. Pat. No. 1,571,048 discloses a process of mineralizing a fibrous material such as sawdust with a solution of a metallic salt. The mineral compound precipitates in and on the sawdust, which when mixed with cement enables the sawdust to firmly adhere to the cement.
In the context of cellulose fibers, the increased bond strength to which the above references are directed is desired because cellulose fibers as found in their natural state are held together with lignin which make it difficult to bond the fiber to cement. However, the teachings of the patents above are not specifically directed to the use of partially delignified and individualized fibers, which generally bond well with cement and therefore would not require such treatment methods. In addition, when held together by lignin, cellulose fibers do not encounter the same degree of water and moisture related damage, such as discussed above, that are encountered when using partially delignified and individualized fibers. This is because lignin is substantially more waterproof than the cellulose fibers within the lignin.
Accordingly, what is needed is a method for preventing water damage and other problems in fiber cement building materials incorporating partially delignified and individualized fibers, and the associated material formulations and products resulted therefrom.
The preferred embodiments of the present invention disclose a new technology, namely cellulose fiber reinforced cement composite materials using loaded cellulose fibers. The cellulose fibers are preferably individualized fibers, wherein at least a portion of the lignin has been removed from the cellulose. Aspects of the technology disclosed include formulations, methods of making the composite materials, and final materials and their properties. This technology advantageously provides fiber cement building materials with the desirable characteristics of reduced water absorption, reduced rate of water absorption, lower water migration, and lower water permeability.
Final products made from these materials have improved freeze-thaw resistance, reduced efflorescence, reduced dissolution and re-deposition of water-soluble matrix components in natural weathering. It is possible, with the proper fiber loading, to improve other product properties, for example, rot and fire resistances, compared to conventional fiber cement products. It has been found, surprisingly, that these improved attributes are gained without loss in dimensional stability, strength, strain or toughness. Even more surprisingly, strength, strain and toughness may even be improved with less cellulose being used than conventional cellulose fiber cement composite materials.
More particularly, Applicant has found that by filling, or partially filling the internal hollow spaces of cellulose fibers with insoluble inorganic and/or organic materials, an engineered cellulose fiber can be produced that, when used in cement composites, still has the advantages of regular cellulose fibers of refining, autoclaving, and manufacture without pressing, but the resultant fiber cement material also approaches or exceeds the performance advantages of artificial fibers such as PVA, in terms of the rate and amount of water absorption when used in fiber reinforced cement composite materials. What is more surprising is that smaller quantities of fibers may be used, so that the cost of loading or partially loading the fiber can be offset by the lower usage of the fiber in products, without a reduction in the important physical properties of the material, such as strength and toughness.
In particular, certain preferred embodiments show that when used in formulations typical of autoclaved cellulose based fiber cement, the rate of water absorption and the amount of water absorption are greatly reduced in the composite product, thus reducing the tendency to efflorescence, or to dissolve and re-deposit chemicals internally to the product, or to suffer freeze/thaw damage.
Also, the fibers may still be refined to act as a catch medium in the Hatschek process, they may still be autoclaved without excessive fiber degradation, and they make products adequate in strength without pressing. Moreover, most surprisingly, even with lower amounts of actual cellulose fiber, the preferred embodiments experience no reduction in key physical properties such as strength, stiffness, toughness and moisture movement, and may, in fact, improve some of these properties, especially toughness.
Thus, the use of engineered loaded fibers imparts to the composite material these enhanced properties, and therefore constitutes an alternative technology that, when fully implemented, has the potential to improve mechanical properties and the workability with the material in building and construction, while improving the durability of the products in various environments including especially those that involve cyclic wetting and drying, fire, freezing and thawing, and exposure to the atmosphere, regardless of the means of manufacture. They are particularly suitable to the Hatschek process that requires a refineable fiber (to catch solid particles) and to the autoclave curing cycle that allows the replacement of cement with fine ground silica, although they may also be of use in the air cured products, in conjunction with PVA, to reduce the necessity of the expensive process pressing.
Accordingly, the preferred embodiments of the present invention relate to a new technology of making fiber reinforced cement composite materials using loaded cellulose fibers. This new technology includes formulations, manufacturing processes and final composite materials. These embodiments will reduce water permeability, water absorption, efflorescence, internal water dissolution and re-deposition of materials, and improve durability in freeze/thaw weathering environments. These can be accomplished while maintaining or improving key mechanical and physical properties, especially toughness, surprisingly with less cellulose fiber than would be used in normal cellulose fiber cement. Moreover, this technology is also beneficial for solving one of the key problems of air cured, PVA reinforced fiber cement, by eliminating the need for the expensive process of hydraulic pressing of the formed xe2x80x9cgreenxe2x80x9d body, to crush the cellulose fibers and reduce water permeability in finished products.
In one aspect of the present invention, a composite building material is provided comprising a cementitious matrix and individualized cellulose fibers incorporated into the cementitious matrix. The cellulose fibers are partially or completely delignified. The cellulose fibers have voids that are at least partially filled with loading substances that inhibit water from flowing therethrough.
In another aspect of the present invention, a material formulation used to form a composite building material comprises a cementitious binder and cellulose fibers, wherein the cellulose fibers have been individualized and wherein at least some of the cellulose fibers are loaded with insoluble substances to inhibit water migration through the fibers. In one embodiment, the building material formulation preferably comprises about 10%-80% cement, about 20%-80% silica (aggregate), about 0%-50% density modifiers, about 0%-10% additives, and about 0.5%-20% loaded individualized cellulose fibers or a combination of loaded cellulose fibers, and/or regular unloaded fibers, and/or natural inorganic fibers, and/or synthetic fibers. The materials from these formulations can be autoclave cured or air-cured.
In another embodiment, a building material formulation is provided for an unpressed, autoclaved, fiber cement product. This formulation comprises about 20-50% cement, more preferably about 35%, about 20-80% fine ground silica, more preferably about 55%. Additionally, about 0-30% other additives and density modifiers may be included in the formulation. The formulation preferably includes about 0.5-20% fibers, more preferably about 10% fibers, of which some fraction of the fibers is individualized cellulose fibers loaded with inorganic and/or organic materials that reduce water flow in the fiber pore space.
The voids in these loaded fibers are partially or completely filled with insoluble substances to inhibit water from flowing through. Preferably, the insoluble substances have substantially the same or similar thermal and moisture expansion coefficients as that of the cement matrix. The insoluble substances may comprise organic compounds, inorganic compounds, or combinations thereof. The loading substances can comprise about 0.5%-200% of the dry weight of the cellulose fibers. Most commonly, loading substances in the loaded fibers are approximately 10%-80% of the cellulose weight.
Another aspect of the present invention relates to a method of manufacturing a fiber reinforced composite building material. The method in one embodiment comprises individualizing cellulose fibers by removing a majority of the lignin binding the cellulose fibers together, sometimes with aids of mechanical forces. At least a portion of the cellulose fibers is loaded with an insoluble substance to form loaded cellulose fibers, wherein the insoluble substance inside the fibers inhibits water flow through the fibers. The loaded fibers are mixed with a cementitious binder to form a fiber cement mixture. The fiber cement mixture is formed into a fiber cement article of a pre-selected shape and size. The fiber cement article is cured so as to form the fiber reinforced composite building material.
The step of loading the fibers preferably comprises loading the fibers with inorganic compounds, organic compounds, or combinations thereof using techniques involving chemical reactions and/or physical depositions. Preferably, the step of mixing the loaded fibers with ingredients to form a fiber cement mixture comprises mixing the loaded fibers with non-cellulose materials such as a cementitious binder, aggregate, density modifiers, and additives in accordance with the preferred formulations of this invention. In another embodiment, the loaded fibers can also be mixed with conventional unloaded fibers and/or natural inorganic fibers, and/or synthetic fibers along with the other ingredients. The fabrication processes can be any of the existing technologies, such as Hatcheck process, extrusion, and molding, etc. Advantageously, in one embodiment the fiber cement article can be autoclaved.
Testing of certain embodiments of fibers with filled voids shows an increase in toughness of the final product by more than about 50%, an increase in the modulus of rupture (MOR) of more than about 15%, and an increase in the modulus of elasticity (MOE) in a bending test by more than about 15%, as compared to a building product made from an equivalent formulation with conventional cellulose fibers. Furthermore, application of the loaded fibers reduces the volume of the pores of the building material in the range of 1-10 micrometers by more than about 30%, more preferably so that the specific pore volume of the fiber cement composites using the loaded fibers is less than about 6 xcexcL/g, measured by MIP (Mercury Intrusion Porosimetry).
Advantageously, the preferred embodiments of the present invention provide fiber reinforced building materials that have reduced water migration, lower water absorption rate, lower water permeability, less efflorescence, less severe dissolution and re-deposition problems, and improved freeze-thaw resistance, strain, and toughness as compared with a building material made from an equivalent formulation without loaded cellulose fibers. Furthermore, the preferred building materials are dimensionally stable and retain the advantages of cellulose fiber reinforced materials. Furthermore, the building material with loaded fibers can be manufactured using conventional processes for fiber cement material. Less cellulose fibers are required in making the composite materials with the enhanced physical/mechanical properties. These and other advantages of the present invention will become more fully apparent from the following description taken in conjunction with the accompanying drawings.